In an optical transmission system, an optical signal is transmitted.
Japanese Laid-open Patent Publication No. 2005-204026, Japanese Laid-open Patent Publication No. 2012-205172, International Publication Pamphlet No. WO 2001/080466, Japanese Laid-open Patent Publication No. 2008-245118, or Japanese Laid-open Patent Publication No. 2005-269194 is an example of related art.